Metal vapor, or ion, laser discharge tubes, although commercially available, have been less than satisfactory when utilized in a machine application. This is particularly so in the cadmium ion laser that employs helium as an auxillary gas, the cadmium ion laser emitting light at either 4116 A in the blue region of the spectrum or at 3250 A in the ultraviolet region of the spectrum and capable of producing a continuous wave or pulsed output with modest discharge currents.
Degradation of the radiant output to an unacceptable level is the usual definition of laser tube failure. In turn, the output radiation varies with the helium and cadmium pressure and the type and concentration of impurities within the tube. The mechanisms for degradation failure in the helium-cadmium laser tubes are therefore related to depletion of the helium or cadmium supply or accumulation of impurities in the tube.
Depletion of the helium supply is, it is believed, usually a result of permeation through the tube seals and glassware, and trapping by the cadmium condensate. In a typical helium-cadmium laser tube with a consumable cadmium supply, such as shown in U.S. Pat. No. 3,878,479, permeation rates have been measured at 0.24 millitorr/hour. In the consumable laser tube designs, typical helium trapping rates by condensing cadmium are as high as 38 millitorr/hour. U.S. Pat. No. 3,663,892 discloses a technique for reducing condensate trapping rates. Depletion of the cadmium supply may be about 1.5.times.10.sup.-2 mg/hr/mA for consumable washer tubes. In recirculating laser tube designs, such as shown in U.S. Pat. No. 3,683,295, the cadmium can be oxidized by dissociated water vapor, thereby depleting the available cadmium for evaporation.
The sources of impurities in laser tubes are both internal and external. Impurities such as hydrogen will increase the conduction and convection cooling losses from the discharge in the capillary bore and thereby cause alteration of the excited state population distribution in the discharge and the laser output.
Water vapor impurities generally dissociate in the tube with the oxygen causing oxidation of the cadmium supply and metal structural components. The residual hydrogen from this dissociation adds to the other sources of hydrogen to cause cooling of the discharge. Internal sources of impurities include absorbed and occluded water vapor and noncondensable gases in the glass walls and structure, evaporated or sputtered electrode or other structural materials, and other internal contaminants resulting from improper cleaning or handling of parts prior to assembly. Suppression or elimination of these problems necessitates that the tube be capable of bake-out at high temperatures (350.degree. to 400.degree. C.) under high vacuum since many of the contamination problems are internal to the tube. External sources of contamination are generally water vapor and hydrogen. The principal location for these contaminants entering the tube is through laser tubes using epoxy seals, the epoxy seal being very porous. Many or all of the mechanisms for degradation failure exist in present commercial laser tubes.
The above deficiencies in the prior art metal ion laser discharge tubes have cause a limited acceptance of these tubes in the laser industry. An important use of a helium-cadmium laser, for example, would be in a laser reading/writing system wherein the blue output of the laser can be effectively utilized as one component of a laser beam which scans (reads) an input document or as a writing beam to write (print) information on a medium sensitive to the blue laser light such as a photoconductor.
Therefore, what is desired in a helium-cadmium laser discharge tube, which, inter alia,: provides an increased lifetime by compensating for helium loss function; provides integral mirror structure to minimize inter cavity optical surface contaminants; provides a laser mirror seal structure which allows bake-out of the tube at the required high temperature to minimize internal tube contaminants; provides a mirror seal structure which is impermeable to leakage of external contaminants; provides diffusion and cataphoretic confinement of the cadmium vapor to protect the laser mirrors; provides a relatively large cadmium reservoir capable of containing sufficient cadmium for lifetimes exceeding 10,000 hours; and, allows for control of the cadmium vapor pressure to maintain a constant laser output indepedent of ambient environment.
It is therefore an object of this invention to provide a laser device of the above noted deficiencies.
It is an object of the present invention to provide a metal vapor laser which has substantially increased operating and shelf life times.
It is a further object of the present invention to provide an integral mirror helium-cadmium metal vapor laser discharge tube which has substantially increased operating and shelf life times than commercially available laser tubes.
It is still a further object of the present invention to provide an integral mirror helium-cadmium laser tube which includes a cadmium reservoir having a volume sufficient to contain enough cadmium for long operating and shelf life times.
It is an object of the present invention to provide a helium-cadmium laser tube wherein the cadmium vapor pressure is actively controllable such that it is substantially independent of ambient temperatures.
It is still a further object of the present invention to provide a helium-cadmium laser tube which incorporates a permeable implant member which contains helium under pressure and which is utilized to compensate for helium pressure loss in the tube whereby operating and shelf life times are substantially increased over commercially available laser tubes.
It is still a further object of the present invention to provide a helium-cadmium laser discharge tube which is compact, economical and has long operating and shelf life times.
It is a further object of the present invention to provide an integral mirror, helium-cadmium laser tube which incorporates both cataphoretic and diffusion confinement sections within the tube to prevent cadmium vapor from condensing on the laser mirrors.